This application is related to copending application Ser. No. 272,105 pending filed Nov. 14. 1988 for "Solar Cell Having Inter-Digitated Contacts and Internal Bypass Diodes", and Ser. No. 272,129 pending filed Nov. 14, 1988 for "Monolithic Series Connected Solar Cells Employing Shorted P-N Junctions for Electrical Isolation".
This invention relates generally to the fabrication of silicon solar cells, and more particularly the invention relates to fabricating back surface point contact solar cells.
The silicon solar cell comprises a plurality of p-and n-conductivity-type regions in a silicon body which generate voltage potential and/or a current when electron-hole pairs are created in a semiconductor body in response to impinging radiation, and the holes and electrons migrate to the p-doped region and the n-doped region, respectively. In a solar cell having interdigitated back surface contacts, the p and n regions are formed in alternating rows with a metal contact provided for contacting all doped regions in one row and with all rows of like doped regions being connected in parallel.
A point contact solar cell has been proposed with the potential for achieving improved energy-conversion efficiency in the neighborhood of 26-28%. See R. M. Swanson, "Point Contact Silicon Solar Cells," EPRI AP-2859 Project 790-2 Interim Report, May 1983. As described therein, in the point contact cell, contact metal touches the silicon substrate in an array of points. The cell is made of lightly-doped silicon with dopant diffusions only at the contact points. The region between contacts, on both top and bottom surfaces, is covered by high-quality silicon oxide for surface passivation. The surfaces are slightly textured and the back side is made reflective to promote light-trapping. Opposite conductivity-type contacts can be on the front and back of a cell, whereupon it is called a front surface cell, or the contacts can be interleaved on the back surface only, whereupon it is called a back surface cell.
As further described by Swanson et al., "Point Contact Solar Cells," IEEE Transactions on Electron Devices, Vol. ED-31, No. 5, May 1984, a back surface cell has been proposed in which a two-layer metallization on the bottom surface provides the metal contacts. Unlike the interdigitated contact cell having alternate p and n fingers, the contact metal touches the silicon only in an array of points on the back surface. The contact areas contain small diffused regions which alternate between n and p type in a checkerboard fashion The top surface and the regions between contacts on the bottom are covered with silicon oxide for surface passivation. The specific design disclosed therein provides low electrical and thermal resistance. The n-type diffusions are contacted by a first layer of aluminum, while p-type diffusions are contacted through holes cut in the first layer. The first and second layers are electrically isolated from each other by a layer of anodic aluminum oxide (A1203).
In fabricating the dual-metal point contact solar cell as disclosed by Swanson et al, supra, a number of processing difficulties have been encountered which have impeded the commercialization of the structures.